As a fundamental method of biomedical research, the fluorescence labeling and detecting technologies play an important role in the studies at subcellular level, cellular level and in vivo level. In in vivo imaging, the fluorescence imaging technology with near-infrared quantum dots has many unique advantages. For example, it has deeper depth of tissue pentration, and can overcome the defect that the deep tissue imaging with visible-light quantum dots is susceptible to the background fluorescence. Thus it draws broad attention in medical diagnostics, molecular biology, cellular biology and the like. Currently, all the common near-infrared quantum dots contain toxic elements such as Cd, Hg, Pb and the like. Silver sulfide (Ag2S) quantum dots with low toxicity or even without toxicity exhibiting near-infrared fluorescence have been reported (Near-infrared photoluminescent Ag2S quantum dots from a single source precursor. J. Am. Chem. Soc., 2010, 132, 1470), but the particles are relatively large and the near-infrared fluorescence intensity is not strong enough. Other literatures regarding Ag2S have not given the report regarding fluorescence. Moreover, the Ag2S reported in those literatures has poor homogeneity and dispersity and is made by complex preparation methods. Furthermore, the surface functionalization of quantum dots, i.e. transformation from hydrophobic form into hydrophilic form, to make the quantum dots to be used for biomedical research, has been reported in many literatures. However, the surface functionalization processes reported is substantially not suitable for Ag2S quantum dots, because all the Ag2S quantum dots have superlattice structure and it is difficult to modify the superlattice with conventional processes. Furthermore, reagents with strong oxidability are not suitable for the transformation of Ag2S to its hydrophilic form. Therefore, it is of great significance to develop a method for preparation and surface functionalization of Ag2S quantum dots, which is simple process and can produce high quality of Ag2S quantum dots with uniform particle sizes, good particle dispersity, high fluorescence intensity and good reproducibility, so that the Ag2S quantum dots can be used in the biological field.